Studies in our laboratory have uncovered a unique approach for increasing the therapeutic index of 5-fluorouracil (5-FU) by combining it with allopurinol (HPP). This 2-year Young Investigator Grant has extended this observation to determine (1) the biochemical mechanism by which HPP antagonizes 5-FU cytotoxicity in certain cell lines (2) if, in tumor-bearing animals, HPP can selectively reduce 5-FU-related toxicity thereby increasing the therapeutic index of 5-FU (3) if it is clinically valuable to combine HPP with 5-FU therapy in order to increase the rate and duration of response of human tumors to this fluoropyrimidine. Based on our reports, others have commenced clinical trials indicating that addition of HPP permits at least a 2-fold increase in the tolerated dose of 5-FU with retention of antitumor activity; a Phase I clinical trial has recently been initiated here in collaboration with the Section of Medical Oncology. Antagonism of 5-FU toxicity by HPP resulted from reduced 5-FU activation to nucleotide form in cells that activate 5-FU via a pyrimidine phosphoribosyltransferase; HPP and other inhibitors of OMP decarboxylase cause the accumulation of intracellular orotate which interferes with 5-FU activation via this pathway. That HPP may selectively reduce 5-FU toxicity to normal tissue but not decrease antitumor activity against malignant tissues was demonstrated in a murine Colon Tumor #38 model system. A two-fold greater dose of 5-FU in combination with HPP doubled the delay of tumor growth compared with an equitoxic, optimal dose of 5-FU alone. A possible explanation is that certain tumors predominantly metabolize 5-FU via a pathway different from that in normal tissues.